Cells of a human acute monocytic leukemia line (THP-1) acquire macrophage-like characteristics following exposure to a diterpene ester, mezerein. Activated cells release both growth inhibitory and stimulatory activities in serum-free medium. Growth inhibition is observed only with malignant target cells while growth stimulation is noted with cells derived from normal tissues and certain malignant lines. The first aim of the current proposal is to purify the factor responsible for growth inhibition using preparative isoelectrofocusing, molecular-sieve chromatography and non-SDS and SDS gel electrophoresis. The second aim is to establish the biological similarities between these activites and other macrophage products such as: interferon, tumor necrosis factor, colony stimulating factor and interleukin 1. The third objective is to confirm target cell specificity with purified inhibitor as a prelude to in vivo studies on tumor response. The final objective is to establish the mechanism of inhibitor action by determining cell cycle dependent sites of action and cell surface binding capacity. The macrophage-monocyte cell series is known to secrete a number of growth mediators many of which are specific for tumor cells. The purification, biochemical characterization and clinical application of many of these mediators has been difficult because either small amounts are produced in vitro or they are produced as part of a complex biological fluid. The current cell system is particularly attractive for the study of macrophage products because: the source is a continuously replicating cell line, inhibitor is produced in a serum-free environment, and preliminary experiments imply that production and purification of large quantities of inhibitor may be accomplished rapidly. The long range goal of this work is to provide a growth inhibitory substance specific for malignant cells.